That's the whole point of the grand jury you're alluding to. They will decide if the facts of the case are worth presenting to a trial jury. This process happens for any criminal case including civilian shootings. I'm not really sure what you think is different here than every other court case tried in the United States. Civilians are let off all the time for shooting other civilians, i.e. home invasion cases, auto theft involving armed robbery where somebody defends their person and property, etc. If a grand jury finds the evidence is insufficient for a case, why should a public servant be held to a different standard than a civilian? Are the rights guaranteed by the Constitution suspended for police officers?
if Darren Wilson was not a police officer, and simply told a kid "Get out of the fucking street", fought with him, then gunned him down and subsequently lied about it and had his co-workers cover for him, we would be justifiably outraged at a murder in which the killer was going to escape justice. so, yes police officers should be given the same rights as civilians and stand for the potential crimes they commit while on (or off) duty.
on the flip side, are the rights guaranteed by the Constitution suspended for young, unarmed black American males? it would certainly appear so based on the vast number of "justifiable" police killings over just this year alone. Mike Brown, Ezell Ford, Kajieme Powell, Eric Garner, Darrien Hunt, John Crawford, Tamir Rice, and the list goes on and on. all killed by police, all unarmed except for Crawford, who had a toy gun he picked up off the shelf of the Walmart he was shot at, and Rice, who was a 12 year old with a toy gun.
if we believe in the Constitution and we also believe in fairly applying the laws and rights conveyed by it, the discrepancy with how it is applied leaves a lot to be desired.
"That's the trouble with political jokes in this country... they get elected!" -- Dave Lippman